In this paper, the problem of joint state and parameter estimation in switching Linear Parameter-Varying (LPV) systems is considered. The proposed solution relies on an adaptive observer, which is designed finding a solution to a system of Trilinear Matrix Inequalities (TMIs). It is shown that, under certain assumptions, the TMIs can be reduced to Linear Matrix Inequalities (LMIs) that can be solved using available software. An example of a four wheeled omnidirectional mobile robot subject to unknown offsets in the motor voltages is used to illustrate the efficiency of the proposed approach.